1. Field of the Invention
The present invention relates to a cutting insert and a rotary cutting tool in which the cutting insert is detachably mounted. In particular, the present invention relates to a cutting insert and a rotary cutting tool which are used to form tooth spaces in a gear.
2. Description of the Related Art
An indexable rotary cutting tool is conventionally used as a cutting tool for manufacture of gears. The tool is formed by assembling, from a side end surface side of a tool body, cutting inserts into insert mounting seats arranged in opposite side end surfaces of the tool body, using clamping screws. The edge shape of the tool is the same as the shape of a tooth space in a gear. When the tool is used to carry out grooving, the edge shape is directly transferred to a workpiece as the shape of teeth.
Japanese Patent Laid-Open No. 2005-66780 discloses an example of a cutting insert for use in such a rotary cutting tool for gear cutting. Now, FIG. 26 and FIG. 27 schematically showing a cutting insert in Japanese Patent Laid-Open No. 2005-66780 are referred to. In a cutting insert 1 in FIG. 26 and FIG. 27, an intersecting ridge portion between an upper surface 2 and each side surface 3, 4 extends to a side portion 5 and is formed as a continuous smooth cutting edge. The side surface 3, 4 is formed to function as a rake face. A lower surface 6 is formed as an insert seating surface. In such a side view as shown in FIG. 27, the intersecting ridge portion between the upper surface 2 and the side surface 3, 4 is formed to appears to protrude outward of the insert. Thus, the upper surface 2 is shaped like a curved surface conforming to the protruding shape of the intersecting ridge portion. On the other hand, the lower surface 6 is flat so as to serve as the insert seating surface. The intersecting ridge portion between the upper surface 2 and each side surface 3, 4 is shaped to have an involute curve shape. Thus, the use of the cutting insert for cutting allows the involute curve shape to be transferred to the workpiece. The cutting insert includes cutting edges formed only on the upper surface side, and two cutting edges enable the involute curve shape to be transferred to the workpiece.
FIG. 28 schematically illustrates, by way of example, that the cutting inserts 1 are mounted on the opposite side end surface sides of the tool body 7 of the rotary cutting tool and that the cutting tool is rotated and is fed to the workpiece to process tooth spaces 8 in an inner gear one by one. FIG. 28 illustrates two opposite cutting inserts 1. However, in actuality, the cutting inserts 1 are displaced from each other by a predetermined distance in a circumferential direction in the tool body.
At present, there is a demand to enable more of the cutting edges of a cutting insert for gear cutting to be used to transfer the involute curve shape to the workpiece, than in the conventional art. This is because at present, an advanced technique and a high precision are required to form cutting edges shaped like the involute curve shape on the cutting insert, resulting in an increase in the manufacturing cost of the cutting insert and thus in the unit price of the cutting insert.
A possible measure for enabling the use of more cutting edges than in the conventional art is to modify the single-sided cutting insert 1 in Japanese Patent Laid-Open No. 2005-66780 into a double-sided cutting insert. The thus modified cutting insert includes cutting edges formed on the lower surface 6 side of the cutting insert 1 shown in FIG. 26 and FIG. 27. Furthermore, not only the upper surface 2 but also the lower surface 6 is shaped to protrude. The cutting insert in this form allows the use of a total of four cutting edges arranged on both the upper and lower surfaces. However, in this case, both upper surface and lower surface, serving as insert seating surfaces, are protruding curved surfaces conforming to the involute curve. To allow the thus shaped cutting insert to be mounted in the tool body, a bottom wall surface of each insert mounting seat of the tool body needs to be shaped like a recessed curved surface conforming to the involute curve. However, substantial labor and cost is required to form such a recessed curved surface on the bottom wall surface of the insert mounting seat using a cutting tool such as an end mill. Thus, such insert mounting seats are unpractical. Furthermore, even if an attempt is made to form the insert mounting seat in the above-described manner, the cutting insert may fail to be sufficiently stably mounted in the insert mounting seat of the tool body because precisely forming such a recessed curved surface on the bottom wall surface of the insert mounting seat is difficult.